This invention relates to a process for making foamed styrene polymers fast cooling during molding by coating silicone surfactants onto the expandable polymer beads by a dry blending process.
The making of low density, cellular, shaped, plastic articles from expandable particles of styrene polymers is well known. Such particles generally contain a blowing agent which boils below the softening point of the polymer and which will cause the particles to expand when they are heated. When the expanded particles are heated in a mold cavity, the particles expand further to fill the mold and fuse together to form a shaped article.
This invention also relates to a process for producing thick-section moldings of foamed styrene polymers.
The formation of molded articles from expandable styrene polymer particles is effected in two steps: (1) pre-expanding of foamable styrene polymer particles to a density of between 1 to 2 pounds per cubic foot; and (2) further heating the pre-expanded particles in a closed mold to cause further expansion and form a fused, one piece article having the shape of the mold. The second step is what is normally referred to as "molding."
The pre-expansion step may be carried out by heating the expandable polymer pellets by any suitable heating medium such as steam, hot air, hot water, or radiant heat. An excellent method of pre-expansion of the particles is a process such as that disclosed in U.S. Pat. No. 3,023,175 by Rodman. Another excellent method is that disclosed in U.S. Pat. No. 3,577,360 by Immel, which teaches the pre-expansion of polymer beads by heating in a dry atmosphere followed by the application of a vacuum to cause the particles to expand to the desired density.
The molding step normally comprises 4 cycles: (1) the preheating cycle in which the empty mold is preheated with steam; (2) the fill cycle in which the mold is filled with pre-expanded polymer particles; (3) the fusion cycle in which the mold is closed and heated with steam to further expand the particles therein and cause the particles to fuse together; and (4) the cool cycle in which the mold is cooled, usually with recirculating water, or the application of vacuum, to cool the polymer below the softening point of the polymer, the mold is opened and the molded foamed molding is removed from the mold.
The molding of large cross-section (thick section) billets of foamed styrene polymers, that are greater than one foot in the smallest dimension, presents many problems not encountered in the molding of thin section parts.
One of these problems is the fact that as the particles expand in the fusion cycle against the hot mold surfaces, the surface of the billet fuses into a solid surface which insulates the core or center portion of the billet from the heating medium. The resulting molded billet thus has a well-fused surface but has decreasingly good fusion as the core of the billet is approached. This decreased internal fusion of the billet is highly undesirable because the core particles tend to break away from one another or crumble, when the billet is cut up into smaller sizes.
A further problem in the molding of large billets is that during the preheat cycle large amounts of steam are required to heat the massive molds. The preheat cycle is done with the vent drains of the mold open to permit the drainage of condensed water from the mold. The preheat cycle is normally necessary because if the mold is cold when the fusion cycle is begun, the steam used for expanding the particles condenses on the surface of the cold mold causing excess water to be present during the fusion cycle. The water thus formed tends to prevent fusion between the particles and to produce water-wet moldings.
The problems of the fusion and preheat cycles can be eliminated by the process of pre-steaming, as described in U.S. Pat. No. 3,855,377.
After the granules have been heated in the mold to form the desired articles, the article must be cooled for a relatively long period of time, depending upon the thickness of the article, before it can be removed from the mold in a self-supporting state to retain its shape outside the mold. Foamed plastic has good insulation qualities, so the cooling time for the article consumes a large part of the molding cycle and greatly restricts the number of articles that can be produced from a mold in a given period of time.
The cooling time is not appreciably shortened by applying cooling media to the surface of the article or to the mold surface since the heat transfer through the article is extremely slow. This low heat transfer rate is in fact one of the properties which makes these articles useful, for example, for insulation. If the articles are removed from the mold too soon, the combination of the softness of the thermoplastic material and the pressure due to the hot interior of the article will cause the article to bulge and thereafter not have the desired shape.